. The ecology of algae : a symposium held at the Pymatuning Laboratory of Field Biology on June 18 and 19, 1959 . other on the stream profile determine the laws of distribution of the animal population as well. Succeeding riffles or shallows frequently carry the greater volume of water on alternating sides of the stream, so that erosion is greater first against the right bank and then against the left. This asym- metrical pattern results in asymmetric distribution of the benthos biocoenoses of the pools, with an accompanying break in their continuity at every riffle . Ulothrix spp . , Stigeocl
. The ecology of algae : a symposium held at the Pymatuning Laboratory of Field Biology on June 18 and 19, 1959 . other on the stream profile determine the laws of distribution of the animal population as well. Succeeding riffles or shallows frequently carry the greater volume of water on alternating sides of the stream, so that erosion is greater first against the right bank and then against the left. This asym- metrical pattern results in asymmetric distribution of the benthos biocoenoses of the pools, with an accompanying break in their continuity at every riffle . Ulothrix spp . , Stigeoclonium tenue and Diatoma vulgare are all characteristic of riffles and regularly drop out as massive components of the vegetation wherever pool conditions obtain. Spirogyra spp., Euglena spp. and other mostly un- attached forms naturally collect where current is minimal. Within a riffle or shallows itself, cer- tain areas are apparently much more favorable than others for the larger algae. Growth of Diatoma vulgare has been observed to be inhibited in the portions of the riffle downstream from large rocks, where water movement is relatively slow. (Fig. 2) Fig . 1 . Diagram of spring zonation of prin- cipal algae on rocks of shallows (average depth ca . 15 cm.) in the Saline River, southeastern Michigan. Dark area represents Ulothrix tenuis- sima filaments, present only on rocks in swift water immediately below the water surface where the current is broken by protruding emergent rocks as at A. Irregular oval patches represent Gom- phonema olivaceum occupying the upstream face of rocks such as A and B in rapid current but slightly below the Ulothrix level. Irregular short lines represent the Diatoma vulgare community which is partly intermingled with the Gomphonema but is dominant at a slightly lower level. Rocks such as C, below a given depth are not colonized by mas- sive Gomphonema colonies or by Ulothrix. Arrow shows direction of water movement. Note that this zonation is not
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